This application generally relates to textured pattern sensing and detection, and more particularly, sensing and detecting using partial-coherence speckle interferometry.
A need exists in the fields of counterterrorism and law enforcement to identify and track suspected terrorists or felons from a distance, without the suspect's knowledge or cooperation, and without leaving a trail that might alert the suspect that he is under suspicion, and by whom. A number of biometric sensing and tracking concepts have been proposed. For instance, remote fingerprinting has been identified as an attractive means for identifying and tracking of terrorists.
A problem, however, in remote, covert fingerprinting of a suspected terrorist or felon lies in the lack of contrast in detecting characteristics of the fingerprint. For instance, there may be insufficient differentiation in the reflectivity, emissivity, or polarization signature between the high points (or ridges) and the low points (or valleys) of the dermal papillae to meet needs under a broad range of conditions.
This problem results because passive sensors typically require some intensity, spectral, polarimetric, or other form of image contrast to differentiate ridges from valleys in the dermal papillae. The subtle natural differentiation based on passive signatures may prove insufficient for discrimination, except under very unusual conditions (e.g., shallow illumination grazing angles adequate to produce shadows). Systems exist that create unnatural differentiation by selectively adding an artificial pigment to either the ridges or valleys. In fact, this is how traditional ink-on-paper fingerprints are taken.
Commercial 3-D Laser Detection and Ranging (LADAR) sensors based on direct detection pulse-echo ranging techniques are presently used for detection variety of 3-dimensional imaging applications. However, they typically do not have the range resolution necessary to measure the submillimeter height difference between ridges and valleys in the texture pattern of a human finger. Moreover, commercial LADAR sensors which can record fingerprints using range contrast require that the subject place his/her finger on a flat glass optical surface or window which provides a flat datum for a “binary” range determination as to whether the texture feature is at the same range as the datum or not. The binary approach can provide sharp fingerprint detail, but requires the suspect to put his/her hand on a flat optical surface or window for scanning and is therefore not effective for covert, remote fingerprint capture.
Laser-based interferometric approaches to high resolution profilometry might also be considered. These require the use of a coherent light source to interrogate the target and the return signal is optically mixed with a coherent local oscillator signal of the same wavelength in a heterodyne detection process. When the return signal and local oscillator signal are in phase, the mixed signal is strong, due to constructive interference. When the return and local oscillator signals are out of phase, the mixed signal is measurably weaker due to destructive interference. Regions of constructive interference in the interferometric image of the finger appear as higher intensity fringes and are separated by the weaker intensity regions of destructive interference. The difference in height from one fringe to the next is precisely the wavelength of the optical signal. This fringe pattern therefore provides a very accurate measure of 3-D surface features, much like a topographic map, where the fringe lines are equivalent to lines of constant surface elevation. This interferometric approach works well when the surface to be profiled is relatively flat so that the fringe lines are separated by more than a pixel and can be distinguished in the image. Unfortunately, this is not the case with fingers when using a light source in the ultraviolet, visible, or infrared regions of the spectrum.
As a result, these conventional approaches have been generally impractical in real-time terrorist-identification scenarios where equipment cannot be pre-positioned, the range is quite variable, and/or the fingerprint must be taken remotely and covertly. The range variability may arise from several factors: the distance between the sensor and the suspect's fingers is not precisely known, the dermal papillae are on a quasi-cylindrical surface and therefore have depth, and/or the target is in motion.